The present invention relates to a NOx sensor for detecting the concentration of nitrogen oxides (NOx) in gas under measurement, such as combustion gas or exhaust gas of an internal combustion engine or combustor, and a production method thereof. Hereinafter, the term “front” refers to a gas sensing side with respect to the axial direction of a NOx sensor and the term “rear” refers to a side opposite to the front side.
In response to the recent tightened automotive emission regulations, there arises a demand to reduce NOx in engine exhaust gases. On this purpose, NOx sensors have been developed for direct measurements of NOx concentrations in the exhaust gases.
The NOx sensor generally includes a sensor element equipped with first and second pumping cells, each of which has an oxygen ion conducting solid electrolyte material and a pair of electrodes arranged on the solid electrolyte material, to define a first measurement chamber in communication with the sensor outside and a second measurement chamber in communication with the first measurement cell. When the exhaust gas is introduced as gas under measurement into the first measurement chamber, the first pumping cell effects its oxygen pumping action against the first measurement cell so as to adjust the oxygen concentration of the gas under measurement in the first measurement chamber to a given level. When the gas under measurement is introduced from the first measurement chamber into the second measurement chamber, the second pumping cell decomposes NOx in the gas under measurement and performs its oxygen pumping action against the second measurement chamber so as to produce an electric current as the result of pumping of O2 dissociated from the NOx in the gas under measurement. The NOx concentration of the gas under measurement can be thus determined based on the second pumping cell current.
However, the NOx sensor (sensor element) cannot obtain adequate sensor characteristics due to insufficient electrode activity just by arranging the electrodes on the solid electrolyte material. In order to attain adequate sensor characteristics, Japanese Laid-Open Patent Publication No. 5-18938 proposes performing aging treatment on the NOx sensor element under a high-temperature atmosphere with the application of a voltage between the cell electrodes. Further, Japanese Laid-Open Patent Publication No. 2004-294079 proposes performing aging treatment on the sensor element under a high-temperature atmosphere and then under a lean atmosphere so as to prevent initial fluctuations in sensor characteristics due to excessive electrode activity.